The present invention relates generally to AC motors and, more particularly, to a system and method for reducing real power usage of open-loop AC motor drive systems (which can include at least a motor drive, a motor, and a connected load).
Open-loop AC motor drives are typically programmed to operate based on a voltage/frequency (V/Hz) operating curve. Because the V/Hz settings of an open-loop motor drive is typically adjustable only while the drive is not in operation and/or the actual motor loading conditions are not known at time of drive commissioning, the V/Hz operating curve is typically a pre-set and static curve that is programmed during the initial installation and setup of the drive. The motor drive may be programmed with a first order or linear V/Hz operating curve to maintain a constant ratio between the voltage and frequency applied to the motor. Such an operating curve maintains a constant flux in the air-gap and, therefore, generates constant torque in the motor. Alternatively, a second order V/Hz curve may be selected, where the output torque is approximately proportional to the square of the motor speed. Some motor drives also provide preset user-programmable V/Hz settings to meet the requirement for special applications. Once programmed, the motor drive typically operates based on the pre-set operating curve during the lifetime of the drive unless an operator changes the drive setting at a later time.
Some drives today have energy saving options, such as “Flux Minimization” or “Flux Optimization.” These options are often designed to dynamically seek a minimal current or flux in the motor, but not the overall motor input power. These options typically select a motor voltage which is between the linear V/Hz setting and the quadratic V/Hz setting.
Typically, the linear V/Hz curve, commonly referred as constant V/Hz curve, is the default setting of the majority of open-loop motor drives. While the linear V/Hz curve settings for a given motor drive are typically programmed to provide constant torque to the load, such constant torque output is not needed for many variable torque applications. Therefore, the V/Hz curve settings often result in wasted energy and inefficient operation of the motor drive system, especially the motor and its connected load.
Furthermore, because the V/Hz curve settings are static, the motor drive operates independently of any changes in operating conditions. Although some motor drives may allow a user to alter the V/Hz curve by, for example, adjusting the start, middle, and/or end points of the V/Hz curve, such programming cannot be performed automatically by the drive itself and may be completed only when the motor is shutdown. This typically requires a skilled technician who has a thorough understanding of the both motor drive and the loading profile of the specific motor and load application.
Another reason why an operator commissioning a drive typically selects a linear V/Hz curve is that the linear V/Hz curve provides constant torque and minimizes the risk that the actual torque using a non-linear V/Hz curve (e.g., second order curve) may not be sufficient to meet the load torque demand for variable torque applications. This is particularly true if the operator commissioning the drive does not fully understand the load profile of the motor application where the drive is installed. For example, in waste water treatment plants, the pump motors are often designed to meet the peak demand when heavy rain occurs in the summer. During the majority of the year, however, the load percentage of the pump motor can be very low. Using a second order V/Hz curve in this application may be risky because the second order V/Hz curve may not provide enough torque when a heavy rain event occurs. Also, the operator who commissions the drive is usually not the same person in a plant who is responsible for monitoring and controlling energy savings. Thus, there may be little incentive for the operator who commissions the drive to select a different V/Hz curve setting for energy savings over the standard linear V/Hz curve settings.
It would therefore be desirable to design a system and method for dynamically adjusting the V/Hz operating curve of an AC motor drive during motor operation to minimize the motor input real power and achieve additional energy savings. It would further be desirable for such a system and method to be capable of responding to changes in the speed reference of the motor drive while maintaining stable motor operating conditions.